Structure of anodic oxide coatings on aluminum

Abstract

Porous surface coatings on aluminum are produced by anodic oxidation in acid electrolytes.[" Pore sizes (10-250 nm) as well as pore densities (10'2-1015 m-') are mainly controlled by the anodizing voltage; the thickness of the layers may reach more than 100 pm and is determined by the amount of charge transferred. The remarkably uniform array of the pores in the centers of the almost hexagonally shaped cells makes the films of interest for application in the field of synthetic membranes and as supports for active materials. [" s] Furthermore, the coatings are widely used as protective layers which can also be colored by impregnation of the pores with organic dyes or by precipitation of inorganic pigments in the pores.[43 After coloring, the pores are closed by treatment with water or steam. This "sealing" process causes the formation of gelatinous boehmite and amorphous oxide hydroxide by reaction of the pore wall material with water.l61The structure of the (amorphous) porous coatings is not clear at present. It is assumed that they are structurally related to the non-porous "barrier-layer" (in direct contact with the metal; thickness 0.1 -0.2 Fm) which consists of almost pure A1,0, and contains the q-and y-phases of alumina (spinel-type structures). This assumption is supported by the primary products of crystallization which are formed by thermal treatment of the porous coatings and which were identified as the y-and y'-phases of alumina.[71 A recent IR-investigation of dried porous layers ['] is consistent with coordination numbers of four and six (tetrahedral and octahedral) for the aluminum atoms.In the present work freshly prepared porous coatings were investigated with X-ray and neutron diffraction methods. The aluminum atoms were found mostly to be surrounded tetrahedrally by four oxygen atoms. The close similarities of the radial distribution function (RDF) of the porous layers to that of vitreous silica suggest that the same structural model should be applied for both materials.Samples for the diffraction experiments were prepared by anodic oxidation of aluminum foils applying the following parameters: bath temperature 12 "C; natural bath convection; current 1. with deviations less than one wt-% for the distinct components. The amount of water given inside the brackets of the formula is regarded as the water of constitution (OH-groups) which is released only at temperatures above 470°C; the amount of water given outside the brackets is regarded as H,O-molecules (structural water and/or adsorbed water at the pore walls) which is already released between 80 "C and 250 "C. Figure 1 (a) shows the X-ray structure factors S" (Q) from angular dispersive X-ray diffraction (ADXD) and from en- ergy dispersive X-ray diffraction (EDXD). Both curves have the same overall shape without sharp reflexes, giving evidence that the anodic oxide coating material is amorphous.In the region of the first peak near Q = 2 A-' the EDXD reflects lower resolution, whereas the high Q-tail of the ADXD curve near Q = 12 k' we judge ...

The Structure of Amorphous and Deeply Supercooled Liquid Alumina

“…The density of amorphous alumina has been reported by several groups (Oka et al, 1979;Kniep et al, 1989;Lee et al, 1995;Wang et al, 1997;Momida et al, 2006;Gorham et al, 2014). Kniep et al (1989) suggested that the density of pure amorphous alumina was 3.3 g cm −3 . Lee et al (1995) reported the density of alumina film deposited by magnetron techniques was 3.4 g cm −3 .…”